My concern here is that I know from past experience that people will do things like:

    client.get_measurements :foo

..without specifying any arguments. That will return `nil` because it isn't valid but they won't get any feedback about it. I think the better approach is to always return a value of the same type (an array) and if that can't be returned (because arguments are invalid, which the no `:count`, no `:start_time` is an example of), some kind of helpful exception should be raised.